Electron microscope with multi-focusing electron lens

ABSTRACT

An electron lens having two pole pieces spaced along a lens axis so as to define a gap between the pole pieces, coil means associated with the pole pieces for, upon being energized, exciting the pole pieces to deflect an electron beam in a manner such that the beam intersects the lens axis at a plurality of points and thereby defining a plurality of focal points along the lens axis when the electron beam emerges parallel to the lens axis. The electron lens also includes an adjustable support means for positioning an object to be imaged.

United States Patent Brookes May 2,1972

[54] ELECTRON MICROSCOPE WITH MULTI-FOCUSING ELECTRON LENS Kenneth A. Brookes, Standon, Ware, Herts, England [72] lnventor:

Associated Electrical Industries Limited, London, England [22] Filed: Nov. 25, 1969 211 Appl.No.: 879,877

[73] Assignee:

[30] Foreign Application Priority Data Nov. 26, 1968 Great Britain ..56,067/68 [52] US. Cl. ..250/49.5 D, 250/495 B, 313/84, 328/228 [51] Int. Cl. ..H01j 37/26 [58] Field of Search ..250/49.5 B, 49.5 D; 313/84; 328/228 [56] References Cited UNITED STATES PATENTS 2,499,019 2/1950 Dornfield ..250/49.5 B

3,508,049 4/1970 Riecke ..250/49.5 D 3,535,514 10/1970 Cardile ....250/49.5 A 3,173,005 3/1965 Suzuki ....250/49.5 D 3,218,457 11/1965 Van Dorsten ....250/49.5 B 3,394,254 7/1968 Le Poole ..250/49.5 D

Primary Examiner-Archie R. Borchelt Artorney-Watts, Hoffmann, Fisher & Heinke [5 7] ABSTRACT An electron lens having two pole pieces spaced along a lens axis so as to define a gap between the pole pieces, coil means associated with the pole pieces for, upon being energized, exciting the pole pieces to deflect an electron beam in a manner such that the beam intersects the lens axis at a plurality of points and thereby defining a plurality of focal points along the lens axis when the electron beam emerges parallel to the lens axis. The electron lens also includes an adjustable support means for positioning an object to be imaged.

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ELECTRON MICROSCOPE WITH MULTI-FOCUSING ELECTRON LENS BACKGROUND OF THE INVENTION This invention pertains to the art of electron lenses and, more particularly, to objective lenses which may be employed in electron microscopes.

Previously known electron microscopes have included vanous types of electron lenses for deflecting and focusing the electrons of an electron beam. These lenses have generally included a pair of pole pieces which are excited by a magnetic coil for establishing a magnetic field between the pole pieces. Electrons entering this magnetic field are then deflected or focused and are transmitted through an object or specimen to be imaged.

These lenses have been operated in a single focus mode of operation, i.e., with the electron beam crossing an axis of the lens only once within the confines of the lens. The specimen or object to be imaged is generally situated on the electron source side of the center of the lens. Since the distance between the pole pieces is relatively small, only limited manipulation of the specimen within this relatively small area is possible. In these known electron lenses, the object or specimen is inserted through an access opening in the lens housing to a position between the pole pieces. In order to permit the extensive manipulation of the object between the pole pieces, the axial gap between the pole pieces must be relatively large. As is generally known, the spherical and chromatic aberration coefficients of an electron lens vary inversely with the magnitude of the magnetic field in the gap; therefore, it is necessary that the coil excitation be increased substantially as the spacing between the pole pieces is increased, in order to maintain a high resolution. There is, however, a limit to the value of the pole spacing at which the lens can operate to produce an image with a given magnetic field when operating in the single focus mode.

SUMMARY OF THE INVENTION The present invention is directed toward an electron lens in which the gap between the pole pieces may be substantially increased without causing an increase in the coefficients of spherical and chromatic aberration of the lens, thereby overcoming the noted disadvantages, and others, of previous electron lenses.

In accordance with one aspect of the present invention, the electron lens includes two pole pieces spaced along a lens axis so as to define an axial gap, means for exciting the pole pieces to deflect the electron beam in a manner so that the electron beam intersects the lens axis at a plurality of points thereby defining a plurality of focal points along the axis when the electron beam emerges parallel to the lens axis, and adjustable support means for positioning a specimen at a focal point of the electron beam.

In accordance with another aspect of the present invention, there is provided an electron microscope having an electron beam generating means for developing an electron beam, and objective lens comprising two pole pieces spaced along a lens axis, one or more magnetizing coils for energizing the pole pieces, an outer housing surrounding and supporting the pole pieces and the magnetizing coils, and means for exciting the magnetizing coils to deflect at least a portion of the electron beam in a manner so that that portion of the beam intersects the lens axis at a plurality of points. The outer housing includes an access opening therein for inserting an object to be imaged, and adjustable support means extending through the access opening in the housing for positioning the object at a selected one of the focal points.

In accordance with another aspect of the present invention, there is provided a method of focusing an electron beam generated by an electron microscope with an electron lens having two pole pieces spaced apart along a lens axis and coil means for exciting the pole pieces. The method steps include: generating an electron beam, exciting the pole pieces with the electron lens in which ample space for extensive specimen manipulation and high resolution are simultaneously obtained in a single lens structure.

Another object of the present invention is to provide an electron lens havinga relatively large gap betweeh the pole pieces, and in which the coefficients of spherical and chromatic aberration of the lens are relatively small.

A still further object of the present invention is to provide an electron lens having a plurality of focal-points, and means for positioning an object to be imaged at a selected one of the focal points.

Another object of the present invention is to provide an electron microscope including an electron lens having a plurality of focal points, and means for positioning a specimen at a selected one of the focal points.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and advantages of the invention will become apparent from the following description of a preferred embodiment of the invention as read in conjunction with the accompanying drawings in which:

FIG. I is a sectional view of an electron microscope including an electron lens illustrating a preferred embodiment of the present invention;

FIG. 2 is a sectional view illustrating in more detail the electron lens of the electron microscope of FIG. 1;

FIG. 3 is a sectional view of a known electron lens illustrating the electron paths;

FIG. 4 is a. graphical diagram illustrating the relationship between focal length and excitation;

FIG. 5 is a graphical diagram illustrating the relationship between aberration coefficient and excitation;

FIGS. 6 through 8 are sectional views of an electron lens in accordance with the present invention illustrating the electron paths of various electron beams.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, FIG. 1 illustrates an electron microscope which generally comprises an electron gun 10, a pair of condenser lenses l2, 14 an object lens 16, three projector lenses 18, 20, 22 and a viewing chamber 24. The electron gun 10 is of known design and generally includes a filament 26 mounted in the upper portion of the electron gun 10 for emitting a beam of electrons. The filament 26 is connected to a conductor 28 which is adapted to be coupled to the negative polarity terminal of a voltage supply source.

The condenser lenses l2, 14, as well as the projector lenses 18, 20, 22 are of known design and are supported relative to each other by an outer housing member 30. Reference is made to US Pat. application Ser. No. 865,869, entitled Electron Optical System," filed Oct. 13, 1969, and assigned to the same assigneee as the present invention, which illustrates in more detail the structural relationship between the electron lenses and the outer housing member 30.

The viewing chamber 24 includes a fluorescent screen 32, positioned beneath projector lens 22, and an observation window 34 for viewing an image on fluorescent screen 32.

FIG. 2 illustrates in more detail the objective lens 16 of FIG. 1. Objective lens 16 generally includes a magnetic circuit 36, two pole pieces 38, 42 extending from magnetic circuit 36, and a pair of magnetizing coils 46, 48 for exciting the pole pieces 38, 42. The magnetic circuit 36 surrounds the magnetizing coils 46, 48 and is positioned within and supported by the outer housing 30. A pair of members 50, 52, are positioned against the surface of coils 46, 48 for retaining these coils within magnetic circuit 36, and the terminals 54, 56, 58,

60 of coils 46, 48 are connected to a current supply source for energizing the magnetizing coils.

The pole pieces 38, 42 extend from magnetic circuit 36 about a lens axis -0, and have lens face positions pole bores 62, 70 extending parallel to the lens axis 0-0, and face portions 64, 72, extending perpendicular to axis 0-0. The lens pole faces 64, 72, define a horizontal gap 78.

Positioned within gap 78 is a specimen or object 80 to be imaged which is carried by a specimen positioning member 82. Positioning member 82 may be translated vertically upon adjustment of a vernier control 84 and is translated horizontally upon adjustment of a vernier control 86. One such specimen positioning arrangement is disclosed in US. Pat. application Ser. No. 85 l,l72, entitled Universal Specimen Stage," filed Aug. 19, I969, and assigned to the same assignee as the present patent application. Similarly, a contrast aperture 88 is positioned within gap 78 and is supported by an aperture positioning member 90 which is translated vertically upon movement by a vernier control 92 and is translated horizontally upon movement of a vernier control 94.

Thus, the specimen or object 80 to be imaged, as well as the contrast aperture 88, may be moved to any selected point within gap 78 upon adjustment of vernier controls 84, 86, and controls 92, 94, respectively.

Reference is now made to FIG. 3 which illustratesan electron microscope objective lens operated in a single focus mode of operation, i.e., the electron beam crosses the lens axis at only one point within the lens. The pole pieces 102, 104, are positioned with respect to each other so as to define a lens gap 110 and a lens axis 0-0. In the single focus mode of operation, the focal point F is determined by the intersection with the lens axis 0-0 of a beam of electrons X-X which emerges from the lens in a direction parallel to the axis 0-0. A beam of electrons Y-Y emerge from the lens in a direction which is not parallel to axis 0-0. This beam of electrons Y-Y' crosses the lens axis 0-0 at a point A more distant from the electron source than the focal point F The intersection point A determines the position at which the contrast aperture 112 is positionedv As illustrated in FIG. 3, only that portion of the lens field which is more distant from the electron source than the focal point F 1 is responsible for imaging an object placed at the focal point F and the focal length of the lens for an object so positioned is designated fo. When, however, the objective lens is employed to image an object outside of the lens field, the focal length would be determined by the entire lens field. In such a case, the focusing characteristics would be determined by the projector focal length f,,.

The relationship between the objective focal point f, and the projector focal length f,,, and the excitation, is described in an article entitled, Imaging Properties ofa Series of Magnetic Electron Lenses, by Liebmann and Grad, Proceedings of the Physical Society, Section B, Vol. 64, 956-971.

FIG. 4 generally illustrates the variation of the objective focal length f, and the projector focal length f as the excitation of the objective lens is increased, where the excitation is expressed in terms of ampere-turns per volt. Thus, it may be seen that the objective focal length 1', decreases as the excitation increases, whereas the projector focal length f exhibits a generally cyclic characteristic.

Accordingly, as the excitation of the lens is increased, the beam of electrons Y-Y in FIG. 3 is deflected in a direction so as to be more parallel with the lens axis O-O, thereby increasing the projector focal length. When the electron beam Y-Y is deflected to a path which is parallel to the lens axis 0-0, the focal length becomes infinite. As the excitation is further increased, the beam of electrons Y-Y is deflected back so that it again intersects the axis 0-0 and the projector focal length f, again becomes finite. This variation in the projector focal length f as the excitation is increased accounts for the cyclic characteristic of the projector focal length as illustrated in FIG. 4. The United States patent to Shigeo Suzuki, US Pat. No. 3,173,005, entitled Magnetic Objective Lens for an Electron Microscope," discloses an objective lens in which the off-axis electron beam crosses the lens axis both before and after the specimen by the intensive converging function of the lens.

When an objective lens is operated in the single focus mode of operation, the projector focal length f, remains in Region I of the curve illustrated in FIG. 4. It has been found that the limiting value of the excitation for an objective lens in the single focus mode of operation is approximately 20 ampere-turns per volt Accordingly, the operation of a lens in a single focus mode creates a restriction on the excitation for a given accelerating voltage.

FIG. 5 generally illustrates the relationship between the aberration coefficients and the excitation. More particularly, as the excitation or the magnetic field within the lens gap is increased, the aberration coefficientdecreases thereby increasing the resolution of the lens. The lens resolution is represented by the equation 8 0.43 C, L where g equals the resolution when C, is the spherical resolution and L is the electron wavelength, the excitation parameter is NI/VR, where NI is the ampere-tums of excitation current and VR is the relativistically corrected accelerating voltage.

It has been found that at normal values of accelerating voltages, high resolution is not compatible with a large lens gap in lenses which are operated in a single focus mode.

If, however, the lens is operated in a multi-focus mode, as illustrated in FIG. 6, the lens gap may be increased while still retaining low coefficients of aberration. In the multi-focus mode of operation, the specimen may be positioned at any of the focal points F F F as illustrated in FIGS. 6 through 8. FIG. 6 illustrates the electron lens operated in a dual-focus mode, the specimen is positioned at the first focus point F and the contrast aperture 88 is positioned at the first aperture point A FIGS. 7 and 8 illustrate the electron lens operated in a tri-focus mode. In FIG. 7 the specimen is positioned at the first focal point F and the contrast aperture 88 is positioned at the first aperture point A,. FIG. 8 illustrates the specimen positioned at the second focal point F and the contrast aperture positions at the first aperture position A With the specimen positioned at the first focus F,, the contrast aperture should be positioned at the first aperture point A,; with the specimen positioned at the second focal point F The contrast aperture may be positioned at aperture points A, or A and, with the specimen positioned at the third focal point F the contrast aperture may be positioned at aperture points A,, A or A It has been found, however, that the minimum values of aberration coefficients are obtained if the object to be imaged is situated on the first focal point F With the specimen situated at the first focal point F the additional intersections are on the illumination side of the specimen and merely increase the length of the prefield. Thus, a multi-focus lens may have a relatively large space between the pole pieces and the specimen, thereby providing sufi'icient space for manipulation of the specimen.

In operation of the electron lens, a specimen is inserted at the desired focal point and then the excitation applied to the lens coils 46, 48 is varied until there is exact coincidence between the desired focal point and the specimen. The variation of the excitation is achieved by varying the resistance of a resistive element in the current supply source. A circuit comprised of resistive elements and a multiposition switch is employed to vary the excitation current in incremental steps in order to selectively vary the number of focal points.

Although the invention has been shown in connection with a preferred embodiment, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention.

Having thus described my invention, I claim:

1. In an electron microscope the combination comprising:

an objective lens comprising two pole pieces spaced along a lens axis with a gap between the pole pieces sufficiently wide to permit manipulation of the specimen in the gap;

means for exciting the objective lens in such a manner that at least two successive focii are simultaneously produced within the lens gap, the first of said focii being of high and the other focus being of lower resolution, said other focus being nearer the center of the lens gap than said first focus; and, means for selectively positioning a specimen selectively and one at a time at either said first focus and said other focus; whereby positioning of the specimen at the first focus will provide a high resolution while positioning the specimen at the other focus will provide a lower resolution but with ample space to enable the specimen to be fully manipulated. 2. The electron microscope of claim 1 wherein said means for exciting the objective lens includes at least one magnetizing coil associated with the pole pieces and positioned with respect to the pole pieces to excite the pole pieces.

3. The electron microscope of claim 1 wherein said means for selectively positioning a specimen comprises a specimen holder which may be positioned at the first focus position for operation with high resolution and may be positioned at other focii positions nearer the center of the lens gap for operation at reduced resolution but with sufficient room for full specimen manipulation.

4. An electron microscope having an electron beam generating means for developing an electron beam; an objective lens comprising two pole pieces spaced along a lens axis so as to define an axial lens gap; at least one magnetizing coil associated with said pole pieces and positioned with respect to said pole pieces to excite said associated pole pieces; an outer housing surrounding and supporting said pole pieces and said magnetizing coils, said outer housing having an access opening therein for inserting an object to be imaged between said pole pieces, said magnetizing coils being adapted for connection to a source of energization whereby at least a portion of said electron beam may be deflected in a manner so that said portion of said beam intersects said lens axis at a plurality of points within the lens gap and thereby simultaneously defines at least two successive focii within the lens gap, the first of said focii being of high resolution and the other focus being of lower resolution, said other focus being nearer the center of the lens gap than said first focus; and adjustable support means extending through said access opening in said housing for positioning a said object selectively and one at a time of either of said focii, whereby positioning of the object at the first focus will provide a high resolution while positioning the object at the other focus will provide a lower resolution but with ample space to enable the specimen to be fully manipu- Iated.

5. An electron microscope having an electron beam generating means for developing an electron beam, an electron lens having two pole pieces spaced along a lens axis so as to define an axial lens gap, means for exciting said pole pieces to deflect said electron beam in a manner so that said electron beam intersects said lens axis at a plurality of points within the lens gap thereby simultaneously defining focii within the lens gap, the first of said focii being of high resolution for another focus being of lower resolution, said another focus being nearer the center of the lens gap than said first focus and adjustable support means for positioning a specimen selectively and one at a time at either of said focal points, whereby positioning of the specimen at the first focus will provide a high resolution while positioning the specimen at the other focus will provide a lower resolution but with ample space to enable the specimen to be fully manipulated.

6. An electron microscope comprising:

a. structure defining an evacuatable microscope column having an elongated electron beam path;

b. an electron source positioned at one end of said path for emitting a stream of electrons;

c. an image-producing mechanism at the other end of said path for producing an image in response to electrons striking said mechanism;

d. electron optical lens means along said path for controlling the flow of electrons in a beam, said lens means defining a lens gap along said path;

c. said lens means being constructed to cause the flow of electrons along said path to simultaneously produce at least three successive focii within the lens gap where electrons cross over the axis of said path, the first of said focii being at higher resolution than the other focii, at least one of said other focii being nearer the center of the lens gap than said first focus; and

f. a specimen stage connectable to said structure and adapted to position a specimen selectively and one at a time at a central one of said other focii in said gap for the conduction of studies at low resolution where substantial room for the specimen or its manipulation is required or at a focus nearer said electron source for higher resolution studies.

7. A method of focusing an electron beam generated in an electron microscope on an object to be imaged with an electron lens having components spaced apart along a lens axis to define a lens gap, and exitation means for exciting said elements, comprising the steps of:

generating an electron beam;

exciting said lens elements with said exitation means to deflect at least a portion of said electron beam in a manner so that said portion of said beam simultaneously intersects said lens axis at at least two locations in said lens gap and thereby defines at least two focii when the said beam emerges from the lens and into the gap, the first of said focii being of high and the other focus being of lower resolution, said other focus being nearer the center of the lens gap than said first focus;

positioning a said object to be imaged at a selected one of said focii along said lens axis within said lens gap; and,

bombarding the specimen with said beam to conduct a stu- 8. A method of focusing an electron beam as defined in claim 7 including the additional step of positioning a said object to be imaged at another one of said plurality of focal points along said lens axis.

9. A method of focusing an electron beam as defined in claim 7 including the step of exciting said pole pieces with said coil means to deflect at least a portion of said electron beam in a manner so that said beam intersects said lens axis at at least three different points along said axis.

10. A method of focusing an electron beam as defined in claim 7 wherein said electron beam generating means is positioned substantially along said lens axis, and including the step of positioning said object to be imaged at a one of said plurality of focal points along said lens axis which is most distant from said electron beam generating means. 

1. In an electron microscope the combination comprising: an objective lens comprising two pole pieces spaced along a lens axis with a gap between the pole pieces sufficiently wide to permit manipulation of the specimen in the gap; means for exciting the objective lens in such a manner that at least two successive focii are simultaneously produced within the lens gap, the first of said focii being of high and the other focus being of lower resolution, said other focus being nearer the center of the lens gap than said first focus; and, means for selectively positioning a specimen selectively and one at a time at either said first focus and said other focus; whereby positioning of the specimen at the first focus will provide a high resolution while positioning the specimen at the other focus will provide a lower resolution but with ample space to enable the specimen to be fully manipulated.
 2. The electron microscope of claim 1 wherein said means for exciting the objective lens includes at least one magnetizing coil associated with the pole pieces and positioned with respect to the pole pieces to excite the pole pieces.
 3. The electron microscope of claim 1 wherein said means for selectively positioning a specimen comprises a specimen holder which may be positioned at the first focus position for operation with high resolution and may be positioned at other focii positions nearer the center of the lens gap for operation at reduced resolution but with sufficient room for full specimen manipulation.
 4. An electron microscope having an electron beam generating means for developing an electron beam; an objective lens comprising two pole pieces spaced along a lens axis so as to define an axial lens gap; at least one magnetizing coil associated with said pOle pieces and positioned with respect to said pole pieces to excite said associated pole pieces; an outer housing surrounding and supporting said pole pieces and said magnetizing coils, said outer housing having an access opening therein for inserting an object to be imaged between said pole pieces, said magnetizing coils being adapted for connection to a source of energization whereby at least a portion of said electron beam may be deflected in a manner so that said portion of said beam intersects said lens axis at a plurality of points within the lens gap and thereby simultaneously defines at least two successive focii within the lens gap, the first of said focii being of high resolution and the other focus being of lower resolution, said other focus being nearer the center of the lens gap than said first focus; and adjustable support means extending through said access opening in said housing for positioning a said object selectively and one at a time of either of said focii, whereby positioning of the object at the first focus will provide a high resolution while positioning the object at the other focus will provide a lower resolution but with ample space to enable the specimen to be fully manipulated.
 5. An electron microscope having an electron beam generating means for developing an electron beam, an electron lens having two pole pieces spaced along a lens axis so as to define an axial lens gap, means for exciting said pole pieces to deflect said electron beam in a manner so that said electron beam intersects said lens axis at a plurality of points within the lens gap thereby simultaneously defining focii within the lens gap, the first of said focii being of high resolution for another focus being of lower resolution, said another focus being nearer the center of the lens gap than said first focus and adjustable support means for positioning a specimen selectively and one at a time at either of said focal points, whereby positioning of the specimen at the first focus will provide a high resolution while positioning the specimen at the other focus will provide a lower resolution but with ample space to enable the specimen to be fully manipulated.
 6. An electron microscope comprising: a. structure defining an evacuatable microscope column having an elongated electron beam path; b. an electron source positioned at one end of said path for emitting a stream of electrons; c. an image-producing mechanism at the other end of said path for producing an image in response to electrons striking said mechanism; d. electron optical lens means along said path for controlling the flow of electrons in a beam, said lens means defining a lens gap along said path; e. said lens means being constructed to cause the flow of electrons along said path to simultaneously produce at least three successive focii within the lens gap where electrons cross over the axis of said path, the first of said focii being at higher resolution than the other focii, at least one of said other focii being nearer the center of the lens gap than said first focus; and f. a specimen stage connectable to said structure and adapted to position a specimen selectively and one at a time at a central one of said other focii in said gap for the conduction of studies at low resolution where substantial room for the specimen or its manipulation is required or at a focus nearer said electron source for higher resolution studies.
 7. A method of focusing an electron beam generated in an electron microscope on an object to be imaged with an electron lens having components spaced apart along a lens axis to define a lens gap, and exitation means for exciting said elements, comprising the steps of: generating an electron beam; exciting said lens elements with said exitation means to deflect at least a portion of said electron beam in a manner so that said portion of said beam simultaneously intersects said lens axis at at least two locations in said lens gap and thereby defines aT least two focii when the said beam emerges from the lens and into the gap, the first of said focii being of high and the other focus being of lower resolution, said other focus being nearer the center of the lens gap than said first focus; positioning a said object to be imaged at a selected one of said focii along said lens axis within said lens gap; and, bombarding the specimen with said beam to conduct a study.
 8. A method of focusing an electron beam as defined in claim 7 including the additional step of positioning a said object to be imaged at another one of said plurality of focal points along said lens axis.
 9. A method of focusing an electron beam as defined in claim 7 including the step of exciting said pole pieces with said coil means to deflect at least a portion of said electron beam in a manner so that said beam intersects said lens axis at at least three different points along said axis.
 10. A method of focusing an electron beam as defined in claim 7 wherein said electron beam generating means is positioned substantially along said lens axis, and including the step of positioning said object to be imaged at a one of said plurality of focal points along said lens axis which is most distant from said electron beam generating means. 